Present day assembly lines employ the technique of spot welding to adhere structural parts together. This technique is used solely because it is the fastest known method. However, spot welding has its drawback in that it creates corrosion problems and also requires an additional sanding step at the point of the weld. There has been a long felt want for an adhesive as a substitute for spot welding. However, to date the adhesives which are of sufficient strength, e.g., epoxies, require curing times of minutes and even hours. Such long curing times preclude the use of these adhesives on assembly lines. Curing times of the order of 10 seconds or less are needed if an adhesive is to replace spot welding. Additionally, the adhesive must have shear strength of the order of 1,000 psi. or more in order to qualify as a structural adhesive.
It is known to use powdered epoxy resins in coatings and adhesives, especially when they are applied electrostatically. Powders, however, have the disadvantage that they cannot be applied in thick sections in applications such as sealants or in certain adhesive joints. Dispersions of epoxy powders in liquid carriers may be applied as thicker beads or ribbons but such mixtures do not cure readily to solid adhesives with the epoxy catalyst which are known in the art, e.g., dicyandiamide, anhydrides and the like. On the other hand, strong amine catalysts such as tertiary amines are useful in two-part epoxy adhesives but they are not useful in one part liquid epoxy adhesives since they impart a very short shelf life thereto.
One object of the instant invention is to produce a one part epoxy adhesive which is storage stable. Another object of the instant invention is to produce a one part epoxy adhesive which results in a thermoset material on heating to temperatures in the range 150.degree.-200.degree. C. Still another object of the instant invention is to produce a method of curing an adhesive dispersion composition comprising a solid epoxy resin containing more than one epoxy group and a solid, sterically unhindered, tertiary amine catalyst. Yet another object in conformity with the aforesaid objects is to produce a method of curing said adhesive composition whereby lap shear strengths of the order of 1,000 psi. are obtained in less than 10 seconds. These and other objects will become apparent from a reading hereinafter.